MOVEMENTS - Cyanobacteria - page 292

A2 genes governing subunits of pili. Amongst extracellular proteins, a glycoprotein designated as
HylA, similar to motility related proteins such as oscillin and SwmA, has been identifi ed. Of the
six genes governing prepilins, at least pilA1 and pilA2 are known to encode proteins with greatest
similarity to bacterial ( Pseudomonas and Myxococcus ) PilA proteins. It was concluded that (i) the
directional motility of Synechocystis depends upon the activity of Tfp; (ii) the cells of Synechocystis
possess different types of pili based on the presence of several components and homologues between
Type II and Type IV pili and (iii) SigF controls the pil genes. These workers envisaged a larger role
for SigF in the adaptation of cyanobacteria to various environmental stresses by its control of cell
surface characteristics and pili-dependent motility. Bhaya et al . (2000) identifi ed the genes involved
in Tfp biogenesis in Synechocystis sp. strain PCC 6803. The major subunit of thick pilus is encoded
by gene pilA1 as evidenced by the fact that mutants of pilA1 are devoid of Tfp or the thick pili.
Genes pilC and pilD govern pilus biogenesis as mutants of these lack both thick and thin pili and
are immotile. This signifi es that both pilC and pilD genes are required for assembly of both types of
pili. Gene pilT1 appears to regulate pilus retraction while pilT2 appears to be involved in phototaxis.
This is evident from the mutants of pilT1 which are immotile but show hyperpiliation and those of
pilT2 exhibit motility but are defective in phototaxis. Yoshihara et al . (2001) identifi ed genes pilA1 ,
pilB1 , pilM , pilN , pilO and pilQ that are required together for motility and transformation competency
in Synechocystis sp. strain PCC 6803. DNA microarray analysis of the target gene for SYCRP1 of
Synechocystis sp. strain PCC 6803 revealed that of the four ORFs ( slr2015 , slr2016 , slr2017 and slr2018 )
only the gene products of the fi rst three exhibited homology to the Tfp pre-pilin from P . aeruginosa
and M . xanthus (Yoshimura et al ., 2002). On the basis these studies, ORFs slr2015 , slr2016 and slr2017
were designated as pilA9 , pilA10 and pilA11 , respectively. It is to be noted that pilA1 to pilA8 genes
were assigned previously by Yoshihara et al . (2001). On the other hand, disruptants for ORF of slr2018
exhibited a non-motile phenotype similar to pil10 and pil11 disruptants reported by Bhaya et al . (2001).
Yoshimura et al . (2002) concluded that not only pilA1 but also other putative pil operon products or
their combination may be required for the structural components of Tfp and cell motility.
Proteomic characterization of membrane proteins of Synechocystis sp. strain PCC 6803 revealed
that PilQ protein ( Slr1277 ) is located in the outer membrane while PilM, PilN, PilO and PilA1 proteins
are located in the plasma membrane (Norling et al ., 1998; Huang et al ., 2002, 2004). PilQ belongs to a
secretin family which is responsible for the formation of a pore for the transport of the pilus across
the membrane (Yoshihara and Ikeuchi, 2004).
Okamoto and Ohmori (2002) determined the biochemical properties of pilT protein after its
purifi cation from E. coli cells in which cyanobacterial pilT gene was expressed. The pilT gene product
also exhibited ATPase activity signifying that pilT is responsible for the generation of the thrust
required for motility and the competence in transformation. A mutant of Synechocystis defi cient in
pilT exhibited a pleiotropic behaviour by the loss of cell motility, increased length of surface pili and
the loss of competence in transformation.
Nakasugi and Neilan (2005) detected the presence of Tfp on the cell surface of the toxic
Microcystis aeruginosa PCC 7806 who identifi ed four genes ( pilA , pilB , pilC and pilT ) governing their
formation. These genes are similar to the already known sequences found in Synechocystis sp. strain
PCC 6803 and other bacteria. Due to the presence of pilT genes sequences in the non-toxic strains of
M . aeruginosa , they concluded that lateral gene transfer might be taking place from toxigenic strains
to non-toxic strains through Tfp-mediated transformation.
To sum up, motility genes in Synechocystis sp. strain PCC 6803 can be grouped into three types:
(i) primary motility genes known as pil genes that are essential for pilus biogenesis and assembly;
(ii) secondary genes that are involved in motility and (iii) pix genes that govern positive phototaxis